Pitot/Static Probllems

[dacker]

This is not a 170 problem but a 172 problem that happened to me but I thought it was interesting and just as applicable to our 170s. I have recently bought a nice little 172 to give flight instruction in and had to take it down to the avionics shop to have an intercom installed. I have had this particular shop do work for me before and have always had excellent service. The installation took up a good portion of the day as expected and seemed to go fine. When I departed I noticed that the airspeed seemed to reach rotation faster than what I have experienced in the ten hours I have flown this airplane but I just shrugged it off. On climbout I also noticed that the VSI was indicating lower than I expected but I just wrote that off to the almost three thousand foot DA that day. About halfway home I noticed that the airspeed indicator was showing 125 KIAS, "H'mm pretty fast 172". I reduced power and watched the airspeed start climbing. I then trimmed the pitch up a little to see if it would slow down... it began increasing. All this time the VSI and altimeter are mostly frozen. The first thing I did was pull the alternate static but that didn't make a difference. It didn't take me too long to figure that things weren't quiete right. By the time I reached Brenham I was indicating 180 kias at 2000 RPM. I made an uneventful full flap landing and put it away for the night.

The next day I began troubleshooting the pitot/static lines (I had thought about it overnight and narrowed it down to the static lines since all three gages were affected. What I found was that when the technician was working he had to pull my ADF out of its tray and when he put it in he pinched the static line that connects to the encoder. When I pulled the ADF out everything unwound. I couldn't feel or see any cuts on the soft rubber line so I put it all back together and went out for a test hop. As I was taxiing I noticed that the AS was indicating 45 kias. I figured maybe it just needed a tap, or ram pressure to jolt it back to normal. This time when I took off the AS increased to 65-70 kias initially but as I climbed to pattern altitude it went to zero. The altimeter and VSI both were frozen. Once again I tried the alternate statit source but no dice.

The only thing I can think of is that another line is pinched (maybe by tie wrap) or it has somehow become blocked. It has to be a static source because only the AS uses pitot pressure.

Any ideas?

David

[GAHorn]

I think you know the problem and the avionics shop has some corrective work to do...at their expense. I'd insist on a complete and thorough pitot/static re-certification. If they cannot perform the work, then they should have it done at their expense elsewhere. (don't forget to charge them for the wasted fuel.)

[dacker]

Well I found the problem today. The static line from the alternate static source was melted. It was tie-wrapped in a bundle near the landing light switch... h'mm, makes me wonder about the connection to the reecent posts about switch problems. I was flying beneath Class B airspace, and it was hazy so I had turned the landing light on while I transited the area.

I'm not sure that I can blame this on the avionics company, it is just a strange coincidence that they also pinched the static line to the encoder (two static line obstructions on the same flight). They refused to send someone to Brenham to fix it and I refused to fly an airplane without airspeed, altimeter, and VSI back to their shop. They did alllow that they would do another pitot/static inspection.

It will be a simple fix, but I wasted several hours troubleshooting the problem. I think that I will also be looking at replacing the landing light switch and beacon switch to prevent any future problems.

David

[voorheesh]

Is it common for a landing light switch or the wiring from it to get hot? My switch (piano key) does not get hot, but the wire underneath gets hot to the touch when the landing light is on. Not hot enough to melt something though. I have the right fuse and it does not blow. Just wondering.

[GAHorn]

David, did the avionics shop "do you a favor" and tie-wrap that static line to the light switch?

I once had a 206 which a repair station worked on. They replaced a connector at the heated pitot head. The next time I used the pitot heat my airspeed became an altimeter and nearly killed me. (I took off at night from a high-elevation, dark-hole airport in the mountains...Sierra Blanca, Ruidoso, N.M. The moment I rotated the runway lights timed out and went black. The take-off direction was towards the mountains and no horizon existed. As I was only a few feet above the surface I became conerned as my airspeed increased beyond the climb speed, so I pitched up. My airspeed continued to increase and I pitched up a bit more, concerned about settling back into the terrain. The stall warning was blaring continuously while I was indicating 100 mph. I decided to fly pitch rather than airspeed and managed to overcome the disorientation caused by the conflicting information in a pitch-black environment.
It turned out that the shop had tie-wrapped the resistance wiring of the pitot-head to the plastic pitot-line, which melted turning the airspeed indicator into an altimeter....so as the aircraft gained altitude during the initial climb...the airspeed gave an indication of too much speed!)

Did they tie wrap your wires to the plastic static line? (Yes, landing light wiring can become hot if marginally-sized for the amperage. Free-standing, non-bundled wires are allowed to be sized smaller than bundled wires. If the shop tie-wrapped them, they may have ignorantly violated the rule.

Even if they did not tie-wrap that circuit.... Did they perform a static-system check after their work? Was the alternate static system already melted from the previous owner? Did they fail to inspect the alternate static source?

Just some things to think about.

Good catch on your part!

[dacker]

The static line was tie-wrapped inside of a bundle that I believe contains the landing light circuit. It was also in close vicinity to the switch. I don't think that the avionics company tie-wrapped the bundle necesarily, but they probably disturbed the placement of the static line. The static line being nylon was probably affected by the heat more than the insulated wiring and melted closed. By the way, this is a bit of a systems learning experience for me. The alternate static source switch is in series with the main static line so that it can only correct a blockage from the external static port on the fuselage to the alternate switch/valve (about 18 inches of tubing). Any blockage after the alternate static source cannot be corrected because the system only uses one common line. The effect was to freeze the altimeter and VSI, and make the ASI act opposite to an altimeter.

I am going to reroute the static lines to the outside and separate to the wiring bundles, and probably replace the landing light switch so as to prevent other heat related occurences.

I have always considered pitot/static blockages to come from icing (which I have had very little experience with), or bugs (I have experienced this). I have never really thought of having a blockage from something internal to the aircraft. That is what keeps aviation so interesting... there is always something new to learn or experience.

Another part of this experience was that this happened on my flight back from the avionics shop right after I decided to switch to the right seat (inflight) in order to practice a few T&Gs. Knowing one of the most important rules in aviation is to "undue what you did last" when you experience a problem, I spent a few moments trying to figure out how switching seats would affect the pitot/static system.

Well, I hope this little discussion will help someone in the future if they have a similar problem in flight.

David